1. Field of the Invention
The present invention relates to a composite plastics sheet having a specific surface resistance of not more than 10.sup.10 ohms and being used as a package material for packing integrated circuitry products. More particularly, the invention relates to a composite plastics sheet comprising a substrate sheet made of polystyrene or ABS base resin having a face covered by a similar resin layer admixed with carbon black particles to have a surface conductivity and being laminated on the substrate sheet by co-extrusion.
2. Prior Art
Polystyrene and ABS base resins are widely used as the materials for molded articles and also as the materials for wrapping sheets. However, the sheets made of polystyrene or ABS resin base plastics materials are apt to be charged with electricity, since they have high specific surface resistances. As a result, when a sheet material made of a polystyrene or ABS base resin is used for packing or wrapping an integrated circuitry (hereinafter referred to as IC) product, there is a risk that the performance characteristics of the IC product are damaged. In order to obviate such a troublesome result, many proposals have been made including, for example, (1) a method of coating an antistatic agent on the surfaces of a package container, (2) a method of coating a conductive paint on the surfaces of a package container, and (3) a method of compounding an antistatic agent or a conductive material, such as carbon black, in the matrix resin.
However, the product prepared by the method (1) set forth hereinabove is not durable for prolonged use, because the covering antistatic agent layer tends to be removed by abrasion or washed away by water. Moreover, the specific surface resistance of the product coated with an antistatic agent is in the order of 10.sup.9 to 10.sup.12 ohms, and the product as such is not suited for use as a package material for wrapping some sorts of IC products, such as LSI (large scale integrated circuitry) products.
The method set forth in (2) above is inconvenient in that the resinous materials which may be used as the vehicles of the paint to be intimately bound to the substrate sheet are strictly limited. Another disadvantage of the product prepared by this known method is that it is difficult to form an even coating of conductive paint. Furthermore, the conductive paint layer is susceptible to rubbing to be peeled off, leading to not only being deprived of the antistatic property but also the damage of the IC products wrapped by the package material coated by the conductive paint layer.
If an antistatic agent is used in the method set forth in (3) above, difficulty is encountered in molding step in case where the added amount thereof is large, whereas the specific surface resistance is lowered in case where the added amount thereof is small. Practically, it is impossible to prepare a sheet having satisfactory static charge preventing property, because the specific surface resistance of the product cannot be lowered less than 10.sup.11 ohms even by the utmost skill. In case where a conductive material, such as carbon black and metal powders, is used as the compounded conductive material, a large amount of carbon black or metal powders must be admixed and compounded with the matrix resinous material to result in deterioration of fluidity at the continuous extrusion molding step to make it difficult to obtain a sheet having satisfactory properties, nevertheless the persistency or durability of the static charge preventing effect and the effect of increasing the specific surface resistance may be greatly improved. In addition, the mechanical strengths, particularly the impact strength, of the resultant sheet material are extremely reduced to an extent unsatisfactory for practical uses due to inferior compounding property. It has been proposed to add a large amount of a variety of liquid paraffin or other mineral oils or a variety of lubricants, or to use a low molecular weight resin, in order to improve the fluidity at the extrusion molding step. However, the sheets obtained by these known methods are unsatisfactory for use as the materials for the IC package containers, for the reason that the mechanical strengths, particularly the rigidity and impact strength thereof are seriously deteriorated.
Further requirements for general sheet materials molded by vacuum or pressure molding processes are that the materials be properly processed through the sheet-forming and trimming steps without any malformation, that the sheet is not fissured during the vacuum and pressure thermo-forming step for molding a container and that any damages, such as formation of cracks, are not caused at the trimming step for finishing the container. In other words, the sheet materials must have the mechanical strengths, particularly the bending and impact strengths, of higher than certain levels.